A mechanically actuated air pressure electric switch is commonly used in commercial and consumer grade electrically powered air compressor applications to monitor and to maintain air tank pressure levels within a pressure band. When tank air pressure is below a predetermined minimum pressure, an electrical contact in the pressure switch is closed in a “cut-in” position to complete an electrical circuit supplying power to an electrical motor that operates a mechanical air compressor connected to the air tank. When tank air pressure reaches a predetermined maximum pressure, the electrical contact within the pressure switch is opened in a “cut-out” position to break the electrical circuit supplying power to the electrical compressor motor. The compressor motor remains off until air tank pressure decreases to the cut-in pressure, when the switch closes the contact again turning the compressor on.
A conventional mechanical air pressure switch for such compressor applications, in one known form, transforms air pressure into switching logic using a pressurized flexible diaphragm that converts air pressure into a mechanical force. The mechanical force acts upon a contact controlling mechanism that toggles an electrical contact between open and closed positions. The pressure driven force input acting on the contact controlling mechanism effects translation within the mechanism of a triggering mechanical element from an initial, non-pressurized position. Translation of the triggering element continues as input air pressure increases until a pre-determined critical translation point at which the mechanism toggles the electrical contact rapidly from the cut-in closed circuit to the cut-out open circuit position. This minimizes electrical arcing during contact opening transition. When pressure driven force input to the contact mechanism decreases, motion of the triggering element reverses as the element moves towards its initial non-pressurized position. During the return travel of the triggering element, a second critical translation point is achieved at which the mechanism toggles the contact back to the cut-in closed position. As is apparent, the cut-in position corresponds to a cut-in pressure, while the cut-out position corresponds to a cut-out pressure. The pressure difference between the cut-out and cut-in pressures is referred to as the switch pressure differential. The contact controlling mechanism that toggles the switch contact is referred to as the switch differential mechanism. The actual cut-in and cut-out pressures are typically determined by use of a large helical compression spring that is pre-loaded to provide resisting force opposing the pressurized diaphragm force. Common manufacturing practice is to adjust the main spring pre-load to determine cut-out pressure. Adjustment of main spring pre-load is typically achieved by compressing the spring through use of a pusher plate and a long adjustment screw of approximately the same length as the main spring free length. The pusher plate is commonly implemented as a metal stamping. The main spring is usually wound from steel wire using standard mass production processes for helical compression springs. Force loads for such springs typically vary over a total tolerance range of about 20%. This variation in main spring load is relatively wide compared to the desired force response of the pressure switch. Calibration or adjustment of switch assemblies is required during manufacturing. The main spring load is adjusted to achieve the desired cut-out pressure. Process time required to pre-compress the main spring for the required pre-load can be significant, as several full rotations of the adjustment screw are often required to set the pre-load.
The present invention is directed to improvements in calibrating pressure switch cut-out pressure.